Hard to find indeed. I think I have dozens of the late style, maybe a half dozen of the transitional version (circa 1921), and a couple of the unribbed plugs. I've only seen the plain ones like the one on the left in pictures.

Ford used Champion and Mosler plugs in 1912. The Champion X plugs for 1912 - 13 have a blood red marking and as mentioned do not have the Patent markings. Champion and Mosler were battling in court over the patent, with the suit ended when Mosler was purchased by Champion.

The Mosler plugs use a different OD porcelain, and the refills from either won't work in the other brand.